1. Field of the Invention
The present invention relates to optical systems and, more particularly, to optical systems that have an unlimited depth of focus.
2. Description of Related Art
A well-known problem encountered with all optical systems is that for any system there is a relatively finite depth of focus, i.e., a fixed range of focus wherein portions of an object or one or more objects are in focus at the same time. A greater depth of focus is desired for presenting to the observer a more complete image of that which is observed. As is well known to those skilled in the art, the depth of focus of a lens system can be increased by increasing the focal length of the lens system and/or using smaller diameter apertures.
In the focus of microscopes, such as used for surgery or biological research, problems arise with using these prior approaches of increasing the depth of focus. In microscopes, there is a practical limit as to the useful length of the lens systems so the focal length is limited. As the magnification increases for a microscope, the depth of focus necessarily decreases, so that with most microscopes the observer is presented with an extremely thin visual image of the observed object that is in focus. When the diameter of the aperture is reduced, this necessarily limits the amount of light that is presented to the observer. To increase the brightness of the visual image, higher intensity lights must be directed at the observed object, either in reflected light systems or for transmitted light systems, which can cause actual harm to the observed object, such as living tissue.
Other methods of increasing the depth of focus of an optical system involve relatively complicated mechanisms to move or rapidly adjust the focus of the optical system, so that the observer is presented with a series of in focus images taken along a plurality of parallel planes passing through the observed object. The trouble with this arrangement is that the depth of focus remains the same, and only the focus is varied manually or by some mechanized means. Some of these prior optical systems are described in U.S. Pat. Nos. 4,448,498; 4,863,252; 5,054,896; 5,321,447; and 5,351,152.